The present invention relates to monomode optical fibers which can be used in particular to compensate dispersion, and it also relates to optical fiber transmission and more particularly wavelength division multiplex transmission using dispersion shifted fiber as line fiber.
The refractive index profile of optical fibers is generally characterized as a function of the curve representing variations in the refractive index as a function of the radius of the fiber. It is conventional to plot on the abscissa axis the distance r to the center of the fiber and on the ordinate axis the refractive index defined in terms of its absolute or percentage difference from the index of the cladding of the fiber. The expressions xe2x80x9cstep profilexe2x80x9d, xe2x80x9ctrapezium profilexe2x80x9d and xe2x80x9ctriangle profilexe2x80x9d are used to refer to curves representative of the index variation as a function of the radius which respectively have step, trapezium and triangle shapes. These curves are generally representative of the theoretical profile or set point profile of the fiber and fiber fabrication constraints can yield a substantially different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplex (WDM) transmission networks, in particular for bit rates of 10 Gbit/s and above; the objective is to obtain substantially zero cumulative chromatic dispersion over the link for all wavelength values of the multiplex or comb, in order to limit widening of the pulses. A cumulative value of a few hundred ps/nm for the dispersion is acceptable. Increasing the unit bit rate of each channel makes it even more necessary to compensate cumulative chromatic dispersion.
Dispersion shifted fibers (DSF) are now commercially available. In these fibers the chromatic dispersion of the transmitted wave is substantially zero at the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 xcexcm at which the dispersion of the silica is substantially zero; silica is compensatedxe2x80x94hence the use of the term xe2x80x9cshiftedxe2x80x9dxe2x80x94by an increase in the index difference xcex94n between the fiber core and the optical cladding. This index difference shifts the wavelength of zero chromatic dispersion; it is obtained by introducing dopants into the preform during its fabrication, for example by an MCVD process well known in the art and not described in detail here. A typical value for the index difference between the cladding and the core of the fiber is 15xc3x9710xe2x88x923; the index of the silica can be increased by doping with geranium, for example. Non-zero dispersion shifted fibers (NZ-DSF) are dispersion shifted fibers which have non-zero chromatic dispersion outside the range of wavelengths used for transmission, typically outside the range of wavelengths from 1520 nm to 1580 nm.
It has been proposed to use dispersion compensating fiber (DCF) to correct the chromatic dispersion induced by a stepped index line fiber. For example, European Patent Application EP-A1-0 554 714 describes dispersion compensating fiber for standard line fibers having a stepped index profile and zero chromatic dispersion at around 1310 nm or flat dispersion fibers having two chromatic dispersion zeros between 1300 nm and 1600 nm. A fiber of this kind has a total dispersion less than xe2x88x9220 ps/nm-km at a given wavelength in the range from 1520 nm to 1565 nm and an attenuation of less than 1 dB/km. For example, the general shape of the profile of fiber of this kind has a narrow central part whose index is higher than that of the cladding, a ring with a higher index than the cladding and a lower index than the central part, and two trenches with the same index, one between said central part and the ring and one between the ring and the cladding. The second trench is optional.
However, when using these DCFs no allowance is made for chromatic dispersion or for the chromatic dispersion slope over the whole of the range of wavelengths concerned. Thus if it is required to compensate only chromatic dispersion, for example, without compensating the slope of said chromatic dispersion, drift occurs in the compensation and the system is not completely compensated at the output since the chromatic dispersion slope is insufficiently compensated. Steps must be taken at the output so that at this end all the wavelengths have the same chromatic dispersion, since otherwise it is necessary to compensate length by length, which quickly becomes prohibitive from both the technical and cost points of view. Accordingly, in this case, it is then necessary to use a greater length of DCF, which induces greater chromatic dispersion and leads to a higher cost.
The fiber of the invention aims to solve this problem of the prior art by enabling correct compensation not only of the cumulative dispersion in the line fiber but also of the dispersion slope. It assures good wavelength division multiplex transmission without inducing high levels of attenuation and it limits non-linear effects. The invention applies in particular to transmission systems using NZ-DSF fibers with RZ, NRZ or soliton pulses.
To be more precise, the invention proposes a monomode optical fiber having at a wavelength of 1550 nm a dispersion slope from xe2x88x920.8 ps/nm2.km to xe2x88x920.5 ps/nm2.km and a chromatic dispersion from xe2x88x92130 ps/nm.km to xe2x88x9290 ps/nm.km, the fiber having a cladding and a core whose index profile features a central part having an index higher than that of the cladding, a ring having an index higher than that of the cladding and lower than that of the central part, and a trench between the central part and the ring having an index lower than that of the cladding.
The line fiber advantageously has no zero value of chromatic dispersion in the range of wavelengths of the multiplex.
Another advantage of the fiber of the invention is that when it is used to compensate the chromatic dispersion of a line fiber the ratio between the chromatic dispersion and the chromatic dispersion slope can be of the same order of magnitude as the same ratio for the line fiber.
In particular, the fiber of the invention generally enables the use of dispersion compensating fiber less than one-tenth the length of the line fiber and preferably less than one-twentieth said length.
The fiber of the invention preferably has a chromatic dispersion slope from xe2x88x920.7 ps/nm2.km to xe2x88x920.6 ps/nm2.km at a wavelength of 1550 nm.
The dispersion compensating fiber advantageously has a chromatic dispersion close to xe2x88x92100 ps/nm.km at a wavelength of 1550 nm.
The difference (xcex94n1) between the index of the central part of the core and the index of the cladding can be from 20xc3x9710xe2x88x923 to 30xc3x9710xe2x88x923 and is preferably around 25xc3x9710xe2x88x923.
The central part of the core advantageously extends out to a radius (r1) from 1.15 xcexcm to 1.25 xcexcm and preferably around 1.2 xcexcm.
The difference (xcex94n3) between the index of the ring of the core and the index of the cladding can be from 0.5xc3x9710xe2x88x923 to 1.5xc3x9710xe2x88x923 and is preferably around 1xc3x9710xe2x88x923.
The ring advantageously has a thickness (r3xe2x88x92r2) from 3.5 times to 4.5 times the radius (r1) of the central part and preferably around four times that radius.
The difference (xcex94n2) between the index of the trench between the central part and the ring and the index of the cladding can be from xe2x88x924.5xc3x9710xe2x88x923 to xe2x88x925.5xc3x9710xe2x88x923 and is preferably around xe2x88x925xc3x9710xe2x88x923.
The trench between the central part and the ring advantageously has a thickness (r2xe2x88x92r1) from 2.5 times to 3.5 times the radius (r1) of the central part and preferably around 3 times that radius.
In one preferred embodiment of the invention the index profile preferably has a trench with an index less than that of the cladding between the ring and the cladding.
The difference (xcex94n4) between the index of the trench between the ring and the cladding and the index of the cladding can be from xe2x88x920.1xc3x9710xe2x88x923 to xe2x88x920.5xc3x9710xe2x88x923 and is preferably around xe2x88x920.3xc3x9710xe2x88x923.
The trench between the ring and the cladding advantageously has a thickness (r4xe2x88x92r3) from 0.8 times to 1.2 times the radius (r1) of the central part and preferably approximately equal to that radius.
The invention also concerns the use of a monomode optical fiber of the invention to compensate dispersion.
Finally, the invention concerns a wavelength division multiplex optical fiber transmission system comprising at least one monomode dispersion shifted line fiber section and at least one monomode dispersion compensating fiber section in accordance with the invention.
In one embodiment of the invention, the line fiber has a chromatic dispersion at 1550 nm from 2 ps/nm.km to 6 ps/nm.km and preferably around 4 ps/nm.km.
In another embodiment of the invention, the line fiber has a chromatic dispersion slope at 1550 nm from 0.06 ps/nm2.km to 0.08 ps/nm2.km and preferably in the order of 0.075 ps/nm2.km.